Nanoparticles for Heat Transfer and Thermal Energy Storage

ABSTRACT

An article of manufacture and method of preparation thereof. The article of manufacture and method of making the article includes an eutectic salt solution suspensions and a plurality of nanocrystalline phase change material particles having a coating disposed thereon and the particles capable of undergoing the phase change which provides increase in thermal energy storage. In addition, other articles of manufacture can include a nanofluid additive comprised of nanometer-sized particles consisting of copper decorated graphene particles that provide advanced thermal conductivity to heat transfer fluids.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/627,749, filed on Sep. 26, 2012, which is incorporated herein byreference, in its entirety, for any and all purposes.

GOVERNMENT INTEREST

The United States Government has certain rights in this inventionpursuant to Contract No. DE-AC02-06CHI1357 between the United StatesDepartment of Energy and University of Chicago Argonne, LLC as operatorof Argonne National Laboratories.

FIELD OF THE INVENTION

The present invention relates generally to the field of heat transferand thermal energy storage. More particularly, the invention relates touse of nanoparticles for improved heat transfer and thermal energystorage. Further, the invention relates to particular nanoparticlearticles of manufacture, including, for example, encapsulated phasechange zinc nanoparticles, silica encapsulated tin nanoparticles andcopper decorated graphene nanoparticles.

BACKGROUND OF THE INVENTION

The demand for highly efficient energy storage and heat transferencompasses a broad range of technologies involving any form of energycreation, storage and usage. In an age of increasing heat fluxes andpower loads in applications as diverse as medical equipment, powerelectronics, renewable energy, and transportation, liquid coolingsystems are necessary to enhance heat dissipation, improve energyefficiency, and lengthen devices lifetime. To satisfy these increasingthermal management needs, the heat transfer efficiency of conventionalfluids must be improved. For example, there is a significant effort todevelop and deploy viable renewable energy technologies. In this regard,for example, solar energy is one of the promising options. However,current costs to produce electricity using solar technologies, such asConcentrated Solar Power (“CSP”), are not cost competitive as comparedto the traditional energy generation technologies based on fossil fuelsand nuclear. Several strategies have been proposed to increase theoverall efficiencies and reduce costs for solar energy production. It isenvisioned that development of high efficiency and high heat capacitythermal storage fluids will reduce the overall thermal storage costs,increase system efficiency, reduce structural storage volume, andcontribute to bringing solar power generation costs in line with otherconventional power generation sources. Particularly, with respect toCSPs, current high temperature energy storage fluids such as moltensalts are relatively limited in terms of their thermal energy storagecapacity. There is therefore a critical need to develop advanced hightemperature fluids (“HTFs”) and thermal storage systems to reduce thecosts and improve efficiencies. Current HTFs, such as synthetic oils,have low thermal conductivity and limited thermal energy storagecapacity. It has been demonstrated in recent years that addition ofsolid nanomaterials to various fluids can increase the thermalconductivity, density, and heat transfer coefficient of nanofluids bytens of percent.

Nanoparticles of functional materials, such as phase change materials(“PCM”), can contribute additional thermal energy storage capacitythrough the latent heat of solid/liquid or solid I/solid IItransformations. Encapsulated in thermally stable and chemically inertshells, PCM nanomaterials dispersed in HTF nanofluids can increasevolumetric thermal storage capacity. Studies of micron-sizedencapsulated phase change materials have been conducted previously forlow temperature heat transfer fluids. The micron sized PCM did notperform well under repeated cycling. The larger particles were oftencrushed during pumping, and the phase change of the PCMs was frequentlyincomplete due to the poor thermal conductivity. Consequently, there isa great need for developing articles and methods for storing energycollected by any means, such as for example, by solar energy methods.

SUMMARY OF THE INVENTION

In one aspect of the invention, coated Zn nanoparticles can be added toan alkali chloride salt eutectic as a phase change material for enhancedthermal energy storage. Zinc nanoparticles (600 nm to about 5micrometers) obtained from a commercial source can be coated withorganic molecules. Thermal cycling tests provide a coated Znnanoparticles with good thermal stability and chemical inertness toalkali chloride salt eutectic during the test cycles (200heating/cooling cycles) in N₂ and in air atmospheres. The volumetricenhanced thermal energy storage of the composite (coated Znnanoparticles/alkali chloride salt eutectic) over the base alkalichloride salt eutectic is about 35 to 40%. Elemental mapping of thecross-sectional view of coated Zn nanoparticles from the composite afterthermal cycles showed no signs of oxidation. In another aspect of theinvention, nanometer sized PCM were used and had significant advantagesas compared with micron sized PCMs. PCMs have higher effective heatcapacity due to their higher phase change efficiency and they are lessrestrictive to flow. In addition, PCMs are less expensive to processwhen fluid based synthesis approaches are used. Therefore addition ofencapsulated PCM nanoparticles with a phase change occurring in a HTF'sworking temperature regime should provide dual advantages: (a) increasedthermal conductivity of a HTF that translates to higher heat transfercoefficient and (b) enhanced volumetric thermal storage capacity of theHTF by latent heat of PCM.

The core/shell nanomaterials synthesized and tested herein were designedfor optimum working temperature range of about 200-250 C with Therminol66 base heat transfer fluid (maximum use temperature of about 345° C.).The core metallic Sn was selected due to the melting point at about 232C and a latent heat of fusion of 60 J/g, while the known stability ofsilica shell in wide range of temperatures was selected to provide thecontainment of the melted tin core. Recent advanced in syntheticchemistry allow synthesis and manipulation of a wide variety ofmaterials at the nanometer scale. Various materials processingtechniques such as sol-gel and thermal decomposition are available toproduce core/shell nanostructures.

Advantageous articles of manufacture were also obtained by addingcore/shell Sn/SiO₂ phase change nanoparticles (Sn/SiO₂ PCNPs) to thesynthetic heat transfer fluid Therminol 66 (TH66). The effects of theSn/SiO₂ PCNPs concentration on thermal conductivity, viscosity, andtotal heat adsorption of the nanofluid suspensions were investigated.Thermal stability of core/shell nanoparticles was also investigated bymultiple heating and cooling cycling and analyzing the morphologicalchanges in PCNPs. In a further aspect of the invention, nanofluids wereengineered by stably dispersing nanometer-sized solid particles inconventional heat transfer fluids at relatively low particle volumeconcentrations to enhance the thermal conductivity and the heat transfercoefficient. In a particular aspect of the invention, hybrid copperdecorated graphene suspensions utilize plasmonic and percolation heattransfer mechanisms to advantage and were able to dramatically boostcooling efficiency.

These and other objects, advantages, and features of the invention,together with the organization and manner of operation thereof, willbecome apparent from the following detailed description when taken inconjunction with the accompanying drawings, wherein like elements havelike numerals throughout the several drawings described below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows an X-ray diffraction pattern of as-received zincnanoparticles with all the characteristic zinc peaks for PDF of Zn card;FIG. 1 b shows the pattern for 600 nm size particle and FIG. 1 c showsthe pattern for 5 micron size zinc particles;

FIG. 2 a shows a scanning electron microscopy image of ˜600 nm sizeas-received Zn nanoparticles and FIG. 2 b shows ˜5μ sized as-received Znnanoparticles, with the particles in FIGS. 2 a and 2 b being sphericalin shape;

FIG. 3 a shows dynamic light scattering of the as-received small Znnanoparticles with an average size above 600 nm and FIG. 3 b shows largesize particles of about 5 μ;

FIG. 4 shows a typical DSC graph of as-received Zn nanoparticles showingmelting and freezing curves;

FIG. 5 shows latent heat of fusion of various 600 nm size Zn samples inN₂ and in an air atmosphere over 20 thermal cycles;

FIG. 6 shows latent heat of fusion of various 5μ size Zn samples in N₂and in an air atmosphere over 100 thermal cycles;

FIG. 7 a shows a TEM image of a composite of TOPO coated Znnanoparticles before thermal cycling and FIG. 7 b shows a TEM imageafter 20 thermal cycles of the composite;

FIG. 8 a shows an SEM image of composite after 200 thermal cycles innitrogen atmosphere with FIB-sliced Zn nanoparticles; FIG. 8 b shows acorresponding elemental mapping (Color code: Zn—“R”, O—“Y”, Al-“G”,Cl—“B”, Ca—“P” and Ba-“M”);

FIG. 9 a shows an SEM image of a composite after 200 thermal cycles inair atmosphere with FIB-sliced Zn nanoparticles, and FIG. 9 b shows acorresponding elemental mapping, (Code: Zn—“R”, O—“Y:, Al-”G”, Cl—“B”,Ca—“P” and Ba-“M”);

FIG. 10 shows a DSC graph of TOPO coated Zn nanoparticles at 11.04 vol %loading in alkali chloride eutectic salt with repeated heating andcooling cycles;

FIG. 11 shows an average % increase in volumetric Thermal Energy Storage(TES) density for 10 Vol % loading of Zn in a salt eutectic as afunction of the temperature range the composite fluid is cycled (ΔT);

FIG. 12 shows X-ray diffraction patterns of as-prepared synthesized tinnanoparticles and silicon oxide coated Sn/SiO₂ core/shell nanoparticles;

FIG. 13 a shows a TEM image of as-prepared tin nanoparticles and FIG. 13b shows a TEM image of silica encapsulated tin nanoparticles;

FIG. 14 a shows a high-resolution TEM image of Sn/SiO₂ PCNP showing acrystalline silica shell with a thick grain boundary between the shelland the tin core; and FIG. 14 b shows an image of uncoated Snnanoparticles showing an amorphous SnO_(x) layer formed on the surfaceof tin core during the sample processing;

FIG. 15 a shows an extended thermal conductivity measurements as ameasure of nanofluid stability with stability of Sn/SiO₂ PCNPsnanofluids in TH66 at various loadings; and FIG. 15 b shows enhancementsin thermal conductivity of nanofluids at different Sn/SiO₂ PCNPsconcentrations compared to the predictions of effective medium theory(EMT);

FIG. 16 a shows viscosity of Sn/SiO₂ PCNPs nanofluids in TH66 as afunction of temperature and FIG. 16 b shows viscosity change with thevolume concentration of nanoparticles;

FIG. 17 shows heat/cool cycling of silica encapsulated tin nanoparticles(“r” line) compared to tin nanoparticles stabilized only with PVP (“b”line);

FIG. 18 a shows volumetric heat absorption of pure TH66 (dashed line),and 5 vol % Sn/SiO₂ PCNPs nanofluids (solid line); and FIG. 18 b showscomparison of the calculated thermal energy storage enhancement with theexperimental data for Sn/SiO₂-TH66 at 5 vol. % nanoparticle loadings;

FIG. 19 a shows a TEM image of Sn/SiO₂ PCNPs before twenty heating andcooling cycles and FIG. 19 b shows an image after twenty heating andcooling cycles;

FIG. 20 shows a schematic of a designed heat transfer nanofluid havingselected thermal enhancement mechanisms (Cu Plasmon resonance andgraphene percolation);

FIG. 21 shows a series of chemical methods of manufacture of hybridCu/graphene nanomaterials and corresponding SEM micrographs of theassociated articles of manufacture;

FIGS. 22 a-22 f illustrates SEM images of various Cu/grapheneparticulates with associated X-ray dispersive imaging of Cu and graphenecomponents;

FIG. 23 a shows a powder X-ray diffraction plot for Cu; FIG. 23 b showsan SEM image; and FIG. 23 c shows an energy dispersive X-rayspectroscopy image of nanoparticles for the Cu-K edge and FIG. 23 d theimage for the Carbon K edge;

FIG. 24 shows increase of thermal conductivity versus volume percentconcentration of nanomaterials versus effective medium theorypredictions; and

FIG. 25 shows thermal conductivity versus temperature for a base fluidand for a 0.5 vol % Cu (graphene nanofluid).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In one preferred embodiment a method and articles of manufacture provideefficient storage of heat energy by concentrated solar power plants(“CSP” hereinafter). In this embodiment a combination is provided of ahigh temperature heat transfer material with inclusion of particulartypes of nanoparticles and also using phase change materials (“PCM”hereinafter) which greatly amplify the ability to efficiently storeenergy. Storage of thermal energy in the form of latent heat in phasechange materials (PCM) is found to be attractive for low and mediumtemperature range applications. Addition of PCMs can contributeadditional thermal energy storage (TES) capacity through the latent heatof solid/liquid or solid I/solid II transformation. The heat transfercan be improved by use of a phase change material (PCM) to transferenergy to steam including the using of composite materials based onhighly conductive graphite. Further, encapsulation methodologies werefound to increase the heat transfer surfaces. Several metal PCMs such asLi, Sn, In, Ga and eutectic alloys have been evaluated and the genericusefulness of such features shown. However, the use of encapsulatedphase change material (EPCM) for high temperature thermal applicationssuch as in CSP is also advantageous. Addition of EPCM nanomaterials toheat transfer fluids (HTF) with a phase change occurring in a fluid'sworking temperature region offers dual advantages: (a) increasedspecific heat of a HTF and (b) enhanced thermal storage capacity of theHTF by latent heat of PCM melting.

Encapsulated Zn Nanoparticles Dispersed in Eutectic Salt.

Thermal stability of coated Zn nanoparticles was determined by thermalcycling (200 heating and cooling cycles) in nitrogen and in airatmosphere and secondly, the benefits determined for adding coated Znnanoparticles to the high temperature alkali chloride salt eutectics forCSP thermal storage applications. The effects of coated Zn nanoparticlessize (600 nm and 5 vt) and total heat adsorption of nanoparticles andtheir suspensions were also established.

Zinc (Zn) was selected as a PCM because the melting temperature of Znfalls in the working range (350-500° C.) of a typical thermal energystorage salt formulations; and it has high heat of fusion (113 J/g)value. Commercial Zn nanoparticles of sizes 600 nm (“small”) and 5μ(“big”) were coated with an organic material which is chemically inertto alkali chloride salt. These composites were dispersed in chloridesalt eutectic at various volumes loading, and the thermal analysis wascarried out using differential scanning calorimetry.

Zinc nanoparticles of two different sizes (<600 nm (small) and 5-10μ(big) were obtained from Sigma Aldrich and their purity was 99%. Thecontainers of the Zn nanoparticles were opened in a glove box containingnitrogen gas, and a small stock of each sample was used to characterizethe as-received particles. The powder X-ray diffraction (PXRD) patternsof as received Zn nanoparticles were recorded by a Bruker D8 X-raydiffractometer with Cu K_(a), radiation. The samples were scanned from2θ of 20 to 80° at increments of 1°/min. The average particle size wereestimated using the dynamic light scattering (DLS) technique at a 90°scatter angle with a 90plus/BIMAS particle analyzer (BrookhavenInstrument Corp, NY).

Morphology of as-received Zn nanoparticles were obtained from scanningelectron microscopy (SEM) and transmission electron microscopy (TEM)(Philips CM 300). The electron dispersive X-ray spectrometer (EDS) datawas obtained on the SEM instrument (Hitachi Model -4700-II, Tokyo,Japan). The samples were prepared by placing a drop of dilute ethanolsuspension of nanoparticles onto a silicon wafer (for SEM) or on to aporous carbon coated copper grid (for TEM) and allowed to dry beforeexamination. Elemental mapping of composite after thermal cycling wasexamined using SEM and the focused ion beam (FIB) technology was used toprepare the cross-sectional view of EPC Zn nanoparticles.

A differential scanning calorimeter (DSC) Q-20 (TA Instruments) was usedto measure the specific heat, heat of fusion, melting andcrystallization of as-received zinc, encapsulated Zn and the compositewith various % volume loadings of EPC Zn nanoparticles. The purge gasesused are nitrogen and air. Before the experiments the instrument wascalibrated against the melting point and enthalpy of melting of indiumand tin. All tests were conducted using a custom temperature programcreated by following the standard DSC test method (ASTM-E1269).

Alkali chloride salt eutectic materials were obtained from theDepartment of Mechanical Engineering, Texas A&M University; and trioctylphosphine oxide (TOPO) reagent plus 99% was obtained from sigma Aldrich.Coating of zinc nanoparticles with TOPO was carried out by the additionof an appropriate amount of TOPO to Zn-nanoparticles, followed by slowmelting of TOPO on to the Zn surface. All sample preparation procedureswere carried out inside the glove box.

The XRD patterns (see FIGS. 1 a-1 c) of as-received Zn nanoparticles(small and big size particles) exhibit prominent X-ray diffraction peaksat scattering angles 2(θ) of 36.2, 38.9, 43.2, 54.3, 70.07 and 70.6degrees. No peaks of oxide impurities were observed in XRD patternconfirming that the obtained Zn nanoparticles are phase pure.

The as-received Zn nanoparticles (small and big) are spherical in shapeas could be seen from the SEM images (see FIGS. 2 a and 2 b). Theaverage size of small particles was around 600 nm and big particles wasaround 5μ, which is in good agreement with the data obtained from DLSmeasurements (see FIGS. 3 a and 3 b).

The melting and freezing temperature, latent heat of fusion ofas-received Zn nanoparticles, coated Zn nanoparticles and coated Znnanoparticles in alkali chloride salt eutectic at various volume %loadings have been studied calorimetrically. The melting and freezing ofas-received Zn nanoparticles was observed at 418.07° C. and freezes at414.54° C. (FIG. 4), which is slightly lower than the reported freezingpoint value for bulk Zn metal (419.6° C.). The measured latent heat offusion of as-received Zn nanoparticles was found to decrease from 97.35J/g to 44.35 J/g (FIG. 5) and 103 J/g to 86 J/g (FIG. 6) for 600 nm and5 size particles respectively after 10 thermal cycles in air. Thesevalues are lower than that of the bulk value reported for Zn metal (112J/g). The difference could be related to the overall surface area of thenanoparticles exposed to oxygen either before or during the thermalcycles. As particle size decreases the surface area increases, so didthe increase in surface oxidation zinc.

Trioctylphosphine oxide (TOPO) is an organophosphorous compound whichcontains three octyl carbon chains resulting in local trigonal symmetry,and has a large permanent dipole moment from the P=0 bond. In industryit has been widely used as a ligand for the extraction of variousmetals, it acts as a solvent and capping molecule in the synthesis of awide range of nanoparticles and semiconductor nanocrystals like CdSequantum dots. It has the ability to form self-assembled monolayers(“SAMs”); and at higher concentration it forms multilayered structuresand protects nanomaterials from oxidation.

As-received Zn nanoparticles were coated with an appropriate amount ofTOPO inside the glove box. Thermal and chemical stability of coated Znnanoparticles have been studied calorimetrically. Stable heat of fusionvalues have been achieved by use of coated particles (both 600 nm and 5μparticles) in both N₂ and in an air atmosphere. Further, these particleswere also stable in alkali eutectic salts at least over the studiedthermal cycles (200). For consistency, only data of 100 thermal cyclesare presented in FIG. 5 and FIG. 6. The TEM images of coated Znnanoparticles before (see FIG. 7 a) and after (see FIG. 7 b) thermalcycles show no changes in the morphology of the particles; and FIBcross-sectional view shows no sign of oxidation.

The latent heat of fusion value obtained for coated 600 nm Zn particleswas 70 J/g and 82 J/g in N₂ and in an air atmosphere (see FIG. 5). Whenthe amount of TOPO was accounted for, the latent heat of fusion valuewas in good agreement with the measured latent heat of fusion value (97J/g) of as-received Zn nanoparticles. For coated 5μ size Znnanoparticles, this value was found to be stable at 97 J/g and 103 J/gin N₂; and in an air atmosphere respectively (see FIG. 6). To furtherdemonstrate the chemical and thermal stability of these coated 5μ sizeZn nanoparticles, these nanoparticles were added to the alkali eutecticchloride salt at different volume loadings and repeated thermal cycles(200) were carried out in both N₂ and in an air atmosphere. Stablelatent heat of fusion values (41 J/g at 21 Vol % in N₂ and 27 J/g at 15Vol % in air) were obtained; and this value is stable over studiedthermal cycles.

FIGS. 8 a and 9 a show the SEM image of FIB cross-section of coated Znnanoparticle after 200 thermal cycles of composite in N₂ and in airatmosphere. The FIB cross-sectional view of coated Zn nanoparticlesshows no change in the particle morphology, densification of particle,and aggregation. Further, there is no observable effect on surfaceoxide.

Elemental mapping of corresponding cross-sectioned particles shows inFIGS. 8 b and 9 b show different elements (O—“Y”, Cl—“B”, Ca—“P”,Al-“G”, Ba-“M” and Zn—“R”) that are present in the composite. Calcium,barium and chlorine are from the eutectic salt, aluminum is from the DSCaluminum pan; and the trace amount of oxygen found next to the Al couldbe from the oxidation of aluminum to alumina. No significant amount ofoxygen was seen around the Zn surface from both thermal cycled samples(composites DSC run in N₂ and in air). It is clearly evident from theSEM and elemental mapping that the coated particles are thermally andchemically stable; and these results supports the stable heat of fusionvalue. However, particles without coating showed considerable change onthe surface texture upon thermal cycles in air atmosphere, which couldbe due to surface oxidation and densification during thermal cycles.

The total heat observed by coated Zn nanoparticles in eutectic alkalichloride salt eutectic (composite) was studied by thermal cyclingbetween 300° C. and 500° C. FIG. 10 shows a typical DSC graph for thecomposite. The two melting and two freezing peaks are from the alkalichloride salt eutectic and coated Zn nanoparticles.

The effective specific heat (C_(p(composite))) and density (ρ composite)of the composite at various temperatures were calculated using the ruleof mixtures (Eq. 1 & 2), and total volumetric heat capacity wascalculated using Eq. 3:

Effective specific heat:

$\begin{matrix}{C_{p{({nf})}} = \frac{{\left( {1 - \phi_{np}} \right)C_{p{(F)}}\rho_{F}} + {\phi_{np}C_{p{({np})}}\rho_{np}}}{{\left( {1 - \phi_{np}} \right)\rho_{F}} + {\phi_{np}\rho_{np}}}} & (1)\end{matrix}$

Effective density of nanocomposite:

ρ_(nf)=φ_(np)ρ_(np)+(1−φ_(np))ρ_(F)   (2)

Volumetric heat capacity:

ΔQ _(PCNF)=ρ_(nf) C _(p(nf)) ΔT+φ _(np) ρ _(npΔH) _(f(PCNP))   (3)

Where C_(p(nf)) is specific heat capacity of mixture(nanoparticles+eutectic salt), C_(p)(F) is specific heat of base salt,C_(p(np)) is the specific heat of Zn nanoparticles, θ_(np) is volumefraction of coated nanoparticles, ρ_(p) is density of base salt, ρ_(np)is density of Zn nanoparticles, ρ_(nf) is the effective density ofmixture, ΔT is the difference in temperature measured from T₁ to T₂ (°C.), and ΔH_(f) (PCNP) is the heat of fusion of Zn NPs.

The measured specific heat of alkali chloride salt eutectic in N₂atmosphere ranged from 1.07 J/g*K at 350° C. to 1.17 J/g*K at 550° C. Ithas an endothermic and exothermic peak at 395° C. and 385° C.,respectively, and the temperature difference between melting andfreezing is 10° C. (see FIG. 10). The specific heat of coatednanoparticles was assumed to be constant in the given range oftemperatures (350° C-500° C.).

A typical DSC graph of composite is shown in FIG. 10 with two endo andtwo exothermic peaks. The endothermic peaks at 395° C. and 418° C.corresponds to the melting from the salt and Zn. The two exothermicpeaks at 385° C. and 414.54° C. correspond to the freezing of salt andZn. The measured specific heat of composite with 8.5 vol % of Znnanoparticles loading was 1.072, 0.889, 0.907 J/g*K at 450, 475, and495° C. respectively for the 1st cycle. The calculated specific heats atthe corresponding temperature are 1.073, 1.047 and 1.024 J/g*K. Thedifference between the measured and the calculated values was found tobe 0 to 18%. After 20 cycles the specific heat value was found to be0.921, 0.874 and 0.854 J/g*K; and the difference between the measuredand calculated value is 16 to 20%. The difference could be a combinationto both the experimental error and the property variation from batch tobatch preparation for alkali chloride salt eutectic. The average heat offusion of Zn was measured to be 17.63 J/g; and the calculated value is22.11 J/g. FIG. 11 shows average % increase in volumetric TES densityfor 10 Vol % loading of Zn in alkali chloride salt eutectic as afunction of the temperature range the composite fluid is cycled (ΔT).The line curve is the predicted value (per Eq. 3) based on the measuredspecific heat of alkali chloride salt eutectic, density of alkalichloride salt eutectic and Zn nanoparticles, and the measured heat offusion of encapsulated Zn. The volumetric TES enhancement is stronglydependent on the temperature range over which the composite fluid iscycled. It should be noted that smaller the (ΔT), the effect of %increase in volumetric TES storage is higher. ΔT increases specific heatof the composite fluid (sensible heat) and is more dominant than theheat absorption from heat of fusion of the melting Zn nanoparticles. Thepredicted increase in volumetric thermal energy storage capacity AQPCNFof alkali chloride salt eutectic with 10 vol, % of coated Zn is ˜40% forthe (ΔT) temperature range of 50° C. and the measured value is 35%. Thedifference is within the experimental scattering. Thus, the totalvolumetric thermal energy storage of the 10 vol % loading of coated Znnanoparticle's show increased TES storage by about 35%.

Thermal stability of as-received Zn nanoparticles was achieved byorganic coating. Further, coated Zn nanoparticles were found to bechemically inert to alkali chloride salt eutectic. Addition of 8-11 vol% loading of coated Zn nanoparticles to salt eutectic will increasethermal storage capacity over the base salt eutectic. Elemental mappingof coated Zn nanoparticles, stable heat of fusion value, absence ofoxide layer over coated nanoparticles from the FIB cross-section valuesupported the stability of coated Zn nanoparticles over repeated thermalcycles in both N₂ and in an air atmosphere.

Encapsulated Tin Nanomaterials in Organic Heat Transfer Fluid

Tin nanoparticles were synthesized using a modified polyole wet-chemicalreduction process. In a typical synthesis, 15 g of polyvinylpyrrolidone(PVP, MW 40000), Sigma Aldrich) was dissolved in 250 mL of tetraethyleneglycol (TEG, 99%, Sigma Aldrich) and heated to 140 C using a heatingmantle. At 140° C., SnCl₂ solution (5 g in 50 mL of TEG) was addedslowly. The PVP solution turned yellow-brown after addition of the SnCl₂solution. Ten minutes later, a freshly prepared NaBH₄ solution (15 g ofNaBH₄ (99% Sigma Aldrich) in 200 mL of TEG) was added drop-wise to thereaction solution. The color of the reaction mixture quickly turned to apale yellow, then to black, and finally to gray upon continued additionof the NaBH₄ solution. After 90 minutes, heat was removed; and thesolution was brought to room temperature. The entire synthesis wascarried out with constant magnetic stirring under the protection of anN₂ atmosphere. The tin nanoparticles were washed three times withethanol and separated by centrifuging. The separated tin particles wereimmediately subjected to encapsulation with silica coating. PVP was usedas a capping agent for the nanoparticles and as an organic template forforming the mesoporous silica coating via base-catalyzed hydrolysis oftetraethyl-ortho-silicate (TEOS).

For silica coating, tin nanoparticles (˜2.9 g, ˜95% yield) weredispersed in 800 mL of ethanol and sonified for 1 hr in a water bath.During the sonication, the suspension temperature increased to 40° C.The flask with nanoparticle suspension was transferred onto a magneticstirring plate and 1 mL of TEOS (99.9%, Alfa Aesar) was added drop bydrop under vigorous stirring. After 45 minutes of stirring, 25 mL ofammonium hydroxide (Fisher Scientific) was added drop by drop to thereaction mixture and stirring continued overnight at room temperature. Awrap of aluminum foil was used to restrict the light source reaching thesystem to restrict photo-polymerization in the reaction flask. Theresulting core/shell Sn/SiO₂ nanoparticles were washed with ethanol anddried in a glove box under flow of nitrogen gas.

Morphology and elemental composition of uncoated tin nanoparticles andSn/SiO₂ PCNPs was characterized using scanning electron microscopy (SEM)equipped with an electron dispersive X-ray spectrometer (EDS) (Hitachi,S4700) and transmission electron microscopy (TEM) (Philips, CM300). Thesamples were prepared by placing a drop of dilute ethanol suspension ofnanoparticles onto a silicon wafer (for SEM) or a porous carbon coatedcopper grid (for TEM) and allowed to dry.

Powder X-ray diffraction (XRD) patterns of nanoparticles were recordedby a Bruker D8 X-ray diffractometer with Cu K_(a) radiation. The sampleswere scanned from 2✓ of 20° to 80° at increments of 1°/min.

Nanofluids were prepared by the addition of Sn/SiO₂ PCNPs at loadings1-5 vol % to synthetic heat transfer fluid Therminol-66 (TH66®)(Solutia, Inc.). Benzalkonium chloride (Acros Organics) was used as asurfactant for dispersing the silica coated particles in TH66 as it wassuggested for dispersing the silica nanoparticles in TH66 [5].Surfactant was first dispersed into the base fluid (TH66), followed byaddition of Sn/SiO₂ PCNPs. The mixture was homogenized by sonicationusing S-450Branson Sonifier at ˜80 W output power, and 40% duty cycle.

The effective thermal conductivity of TH66, TH66 with surfactant, andTH66 with surfactant and various loadings (1-5 vol. %) of Sn/SiO₂ PCNPswere measured using the transient hot-wire technique (KD2 Pro, DecagonDevices). The reported values represent the average of at least 20measurements.

The viscosity of the nanofluids was measured as a function oftemperature in the range between 15 and 135° C. using a BrookfieldDV-II+ rotational type viscometer with the SC4-18 spindle (instrumenterror ˜2%).

The total heat adsorption by nanofluids and pure materials were measuredby differential scanning calorimeter (DSC, Q-20 by TA Instruments). TheDSC instrument was calibrated using an indium standard with themeasurements conducted under the flow of high purity nitrogen gas (N₂).The melting temperature and latent heat of fusion of both uncoated tinnanoparticles and core/shell Sn/SiO₂ PCNPs were measured first. Theempty DSC hermetic aluminum pan and lid were weighed and thennanoparticles were loaded (˜5 mg sample size) into the hermetic aluminumpan. The pan was crimp sealed inside a glovebox (N₂ atmosphere),re-weighed, and placed in the DSC sample holder. Prior to themeasurement, the system was equilibrated at 35° C.; hearing and coolingcycles were between 35° C. and 250° C. at a rate of 10° C./min. Forspecific heat measurements of TH66 and Sn/SiO₂ nanofluids, a customtemperature program that follows the standard DSC test method(ASTM-E1269) as created: the temperature was equilibrated at 50° C. forat least 4 min to evaporate any absorbed moisture, then ramped to 285°C. at 10° C./min. and held isothermal for another 4 min. The cooling toroom temperature was conducted at 10° C./min. rate.

The as-prepared tin nanoparticles synthesized by the polyole reductionprocess yielded a phase pure tin confirmed by powder XRD analysis (seeFIG. 12, middle pattern). The XRD patters of as-synthesized tinnanoparticles exhibit prominent diffraction peaks characteristic for thebody centered tetragonal crystalline phase of tin. The XRD spectra ofcore/shell Sn/SiO₂ nanoparticles (FIG. 12, bottom pattern) show 2additional peaks, which are characteristic of crystalline silicon oxide(FIG. 12, top XRD pattern). No apparent oxidation of Sn occurs duringthe coating process.

FIGS. 13 a and 13 b show the TEM images of as prepared Sn and core/shellSn/SiO₂ nanoparticles respectively. Sn nanoparticles are typicallyspherical shape with particle sizes ranging from 60 to 100 nm (FIG. 13a). TEM of encapsulated nanoparticles (FIG. 13 b) show silica shells asa distinct phase that is evenly coating Sn cores with thickness thatappears to be independent of the core diameters.

The high resolution TEM image (FIG. 14 a) clearly shows the core/shellstructure of Sn/SiO₂ nanoparticles by the contrast in the core and theshell material. The silica shell is crystalline with thickness of ˜5 nm,which indicates that the nucleation of silica was initiated from thesurface of metallic tin nanoparticles. The boundary layer between thesilica shell and the Sn core appears as dense and grainy (FIG. 14 a).The silica coating ensures that the particle core structure maintainsits chemical and structural integrity during exposure to elevatedtemperatures, restricts the leaking of molten metal during the heatingcycles, and allows for a recrystallization process within the confinedshell.

Sn nanoparticles that were not subject to the silica coating (FIG. 14 b)tend to oxidize on the air during the handling process. Uncoated Snparticles revealed a 2-3 nm amorphous SnO_(x) layer at the interface ofthe crystalline tin core. No such amorphous oxidation layers were foundon silica coated particles, where silica coating was applied immediatelyafter the synthesis of Sn nanoparticles.

Stable and homogeneous dispersions of Sn/SiO₂ PCNPs in TH66 wereprepared by the addition of belzalkonium chloride (BAC) cationicsurfactant followed by ultrasonification. The amount of surfactant wascalculated from the optimized surfactant-to-nanoparticle ratio correctedfor the surface area of 100 nm average diameter of Sn/SiO₂ PCNPs.Therefore 0.56 g, 1.12 g, 1.68 g, and 2.8 g of BAC were added to 40 mlof TH66 for 1.0, 2.0, 3.0 and 5.0 vol % loading of core/shellnanoparticles.

The thermal conductivity of nanofluids with 1-5 vol % loadings weremeasured and compared to the values of pure TH66. The thermalconductivity values were recorded automatically every 15 min over 15hours. Nanoparticle settling typically results in a decrease in thermalconductivity values. The consistent thermal conductivity readings at allnanoparticle concentration over an extended time period (see FIG. 15 a)allows us to reach conclusions about the stability of prepared Sn/SiO₂PCNPs nanofluids in TH66. The thermal conductivity of TH66 with onlysurfactant was also tested, and no significant change in the thermalconductivity value was found due to the surfactant. Therefore, theeffect of surfactant on the thermal conductivity of nanofluids can beexcluded. Thermal conductivity of Sn/SiO₂ PCNPs nanofluids showed nearlylinear increase with particle volume concentration (see FIG. 15 b),following the prediction of the effective medium theory (EMT) forspherical particles (˜3% increase in thermal conductivity for eachvolume percent of the solid particles added to the fluid). No additionalenhancement in the thermal conductivity over the EMT predictions wasobserved, as it has been reported in the art for some nanofluids withmetallic particles.

The viscosities of pure TH66 with surfactant and nanofluids with variousloadings of Sn/SiO₂ PCNPs were measured between 25° C. and 125° C. (seeFIG. 16 a). The viscosity increases with concentration f nanoparticles(see FIG. 16 b), and for the same particle concentration decreases withincrease in temperature (see FIG. 16 a). Relative increase in viscosityof nanofluids also decreases with increasing temperature, i.e., athigher temperatures, nanofluids flow more like the base fluid TH66. Atroom temperature, viscosity of 5 vol % Sn/SiO₂ PCNPs nanofluid wasroughly an order of magnitude higher than viscosity of base fluid, whileat 125° C. the viscosity of nanofluid and base fluid are quite similar.Although the viscosity of nanofluids decreases with temperature, thepumping penalty on the nanofluid may be higher than heat transfer andthermal storage benefits of adding nanoparticles. Therefore, thepotential use of nanofluids for optimized thermal managementapplications may require some modest experimentation in terms of volumeloadings of the nanoparticles.

The melting temperature and latent heat of fusion of both Sn/PVPstabilized particles and Sn/SiO₂ PCNPs have been studiedcalorimetrically (see FIG. 17). The melting of Sn cores was observed at228.3° C., while freezing occurred only at 98.3° C. Such large thermalhysteresis (undercooling) in tin melting/freezing (˜130° C.) could berelated to antifreezing properties of PVP shell or due to the small sizeof the high-purity Sn particles. Similar thermal hysteresis has beenreported in the prior art for 5 nm Pb particles in SiO₂ matrix. This isinterpreted as arising from the large undercooling of PB particles as aheterogeneous nucleation process in the nearly impurity-free fineparticles.

The latent heat of fusion was measured to be 58 U/g for Sn/PVPnanoparticles and 48 J/g for Sn/SiO₂ PCNPs, which is lower than that ofthe bulk value reported for Sn metal (60 J/g). The difference betweenexperimental and prior art values is likely due to the mass contributionof PVP and silica coatings that are inert in the tested temperaturerange (50° C.-285° C.). The mass fraction of tin in Sn/PVP wasdetermined to be 97%, while Sn/SiO₂ PCNPs were composed of 8% of tincorrespondingly.

Total heat absorbed by Sn/SiO₂ nanofluids in TH66 was studied by thermalcycling between 50° C. and 285° C. FIG. 18 a shows the truncated DSCcurves for the pure TH66 and the composite nanofluid. Clearly, themelting of tin core is observed in the composite nanofluid at ˜232° C.

The effective specific heat (C_(p(nf)) ) and density (er′_(nf)) of theSn/SiO₂ nanofluids at various temperatures were calculated using therule of mixtures (Eqs. 4 & 5), and total volumetric heat capacity wascalculated using Equation 6:

Effective specific heat:

$\begin{matrix}{C_{p{({nf})}} = \frac{{\left( {1 - \phi_{np}} \right)C_{p{(F)}}\rho_{F}} + {\phi_{np}C_{p{({np})}}\rho_{np}}}{{\left( {1 - \phi_{np}} \right)\rho_{F}} + {\phi_{np}\rho_{np}}}} & (4)\end{matrix}$

Effective density of nanofluid:

ρ_(nf)=φ_(np)ρ_(np)+(1−φ_(np))ρ_(F)   (5)

Volumetric heat capacity:

ΔQ _(PCNF)=ρ_(nf) C _(p(nf)) ΔT+φ _(np)ρ_(np) ΔH _(f(PCNP))   (6)

Where C_(p(nf)) is specific heat capacity of mixture(nanoparticles+surfactant+TH66), C_(p (F)) is specific heat of basefluid, C_(p(np)) is the specific heat of nanoparticles, φ_(np) is volumefraction of nanoparticles, ρ_(F) is density of base fluid, ρ_(np) isdensity of nanoparticles, ρ_(nf) is the effective density of nanofluid,ΔT is the difference in temperature measured from T₁ to T₂ (° C.), andΔH_(f(PCNP)) is the heat of fusion of Sn/SiO₂ PCNPs.

The measured specific heat of TH66 ranged from about 2 J/gK at 100° C.to about 2.7 J/gK at 280° C. The reported values by the manufacturer are1.85 J/g-K at 100° C. and 2.5 J/gK. There is about 8-10% difference inour measurements as compared to the reported values. This differencecould be a combination to both the experimental error and the propertyvariation from batch to batch for TH66. The temperature dependence ofdensity of the base TH66 fluid and Sn/SiO₂ PCNPs were assumed to beconstant in the given range of temperatures (50° C-285° C.).

No endothermic or exothermic peaks were observed for pure TH66 or theTH66/surfactant base fluid in the studied temperature range. Nanofluidswith Sn/SiO₂ PCNPs have shown both endothermic and exothermic peaks dueto melting and re-crystallization of tin (see FIG. 17). FIG. 18 b showsthe volumetric thermal energy storage (or increased absorption) for the5 vol % Sn/SiO₂ PCNPs over the base TH66 as a function f the temperaturerange the fluid is cycled (ΔT). The dashed curve is the calculatedvalues (per Eq. 6) based on the measured specific heat of the TH66,density of TH66 and Sn/SiO₂ nanoparticles, and taking the heat of fusionfor tin as 59 J/g. It should be noted that the volumetric thermal energystorage enhancement is strongly dependent on the temperature range overwhich the fluid is cycled. The larger the (

T), the effect of specific heat of the fluid (sensible heat) is moredominant than the heat absorption from heat of fusion of the melting tinnanoparticles.

The measured volumetric thermal energy storage capacity ΔQ_(PCNF) TH66with 5 vol, % Sn/SiO₂ PCNPs is 408±0.42 J/cc for the temperature rangeof 170° C. The calculated value for the same system is 383 J/cc. Thus,the measured value is about 6.5% higher than the estimation. One of thepossible reasons for this discrepancy may be because the uncertainty inthe total amount of tin in the fluid. Nevertheless, the total volumetricthermal energy storage of the 5 Sn/SiO₂ PCNP's show increased storage byabout 11%. If the reversibility of Sn melting/freezing is improved, thecycling temperature range can be narrowed closer to the melting point ofSn core, thus increasing contribution of volumetric thermal energystorage from the melting of Sn.

The thermal stability of Sn/SiO₂ PCNPs was investigated by conducting 20heat/cool cycles between 100° C. and 270° C. TEM images of Sn/SiO₂ PCNPswere taken before and after this cycling (see FIG. 19). Individual Snspheres remain intact, coated with continuous SiO₂ shell, which confirmsthe thermal stability of core/shell Sn/SiO₂ structures.

Addition of core/shell Sn/SiO₂ PCNPs to high temperature synthetic heattransfer fluid advantageously enhances the thermal conductivity of thebase fluid, while also increasing the total heat capacity. The ceramicshell protects Sn cores from oxidation, thereby providing long-termstability of Sn containing nanofluids. On the other hand, encapsulationof Sn cores allows repeated thermal cycling and prevents coalescenceinto larger size Sn cores when they are melted. Reversiblemelting/freezing of nanoparticle cores is desired for higher enhancementin thermal storage capacity. The thermal hysteresis observed in currentSn/SiO₂ PCNPs can be minimized by introducing impurities to the Sn coreand/or by illuminating PVP layer from the nanomaterials structure.Because the large undercooling, Sn may not be the best core material forpractical applications. In such a large temperature range the overallthermal energy storage will be dominated by the specific heat of thebase fluid. The most benefits from PCM latent heat of fusion will beobtained in a most preferred embodiment when nanofluid is used in anarrow temperature window. Thus, the general criteria for selection ofappropriate core/shell PCNPs are high heat of fusion, reversible meltingand crystallization temperatures, and table shell materials. Addition ofwell-engineered phase change nanomaterials to high temperature heattransfer fluid promises dual functionality in heat transfer and thermalstorage capacity.

Silica encapsulated tin nanoparticles have been synthesized using a costeffective modified sol-gel process. Addition of core/shell Sn/SiO₂nanoparticles to commercial heat transfer fluid (HTF) Therminol 66increased the thermal conductivity of the fluid as predicted from theeffective medium theory, and also improved the total heat absorption ofthe fluid due to the contribution of latent heat of fusion, when tinnanoparticles melted inside of the silica capsules. Thermal stability ofcore/shell nanomaterials during heat/cool cycles was confirmed with DSCmeasurements and TEM imaging. Dual functionality of nanofluids withceramic encapsulated phase change nanoparticles was demonstrated. Betterheat transfer and thermal storage properties of engineered HTFsnanofluids versus conventional HTFs carry a great potential to increasesystem efficiency, reduce structural storage volume, and contribute inbringing solar power generation costs at CSP plants in line with otherconventional power generation sources.

Hybrid Copper Decorated Graphene Nanomaterials with Advanced ThermalConductivity

As shown in FIG. 20, a simple, low cost, and up-scalable wet chemicalsynthesis method was developed for hybrid copper/graphene nanomaterials.The hybrid nanomaterials system were designed as in FIG. 21 to takeadvantage of Cu plasmon resonance and graphene percolation to provideenhanced thermal conductivity characteristics. Variations in synthesisprocedures and reaction conditions resulted in different morphologies ofhybrid nanonomaterials, including the purity of Cu phase, particlesizes, nucleation density, and homogeneity of Cu nanoparticledistribution. Use of SEM and TEM allowed optimizing the synthesisprocedure and achieving the desired nanomaterial morphology to producesuperior heat transfer performance. As shown in FIGS. 22 a-22 f avariety of particle morphologies and structures can be provided forenhanced thermal property applications.

Purity of metallic Cu phase was confirmed with powder X-ray Diffraction(see FIG. 23 a), while atomic distribution of elements in hybridnanomaterials was characterized with SEM and Energy-Dispersive X-RaySpectroscopy (see FIGS. 23 b-23 d): Hybrid Cu/Graphene nanomaterialswere dispersed in synthetic heat transfer fluid Therminol®59 at variousvolume concentrations. Stable nanofluid dispersions were achieved withcombination of two surfactants: octadecane thiol and benzalkoniumchloride. Thermal conductivity of Cu/Graphene dispersions was assessedas a function of volume concentration and temperature. As shown in FIGS.24 and 25, significant increase in thermal conductivity above theeffective medium theory, and graphene suspension of the sameconcentration was observed. Increase in thermal conductivity withincreasing temperature is also indicative of engagement of additionalheat transfer mechanisms (i.e. percolation and plasmonic resonances) aswas designed by nanofluid engineered approach and morphology ofnanomaterials. Nanofluids with copper decorated graphene sheets wereproduced. The selection of nanomaterials was achieved by projectedbenefits from percolation heat transfer mechanisms (see FIG. 21) ofgraphene sheets, phonon resonance heat transfer mechanism of metallicnanoparticles, and synergetic effects of combining Cu and graphene.Besides, Cu nanoparticles attached to graphene sheets provide spacialseparations and prevent them from agglomeration, which is known to bedetrimental for the phonon resonance mechanism. Engineered hybridCu/graphene nanofluids have demonstrated advanced thermal conductivitysignificantly higher that the effective medium theory predictions, andgraphene nanofluids.

The foregoing description of embodiments of the present invention havebeen presented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments, and with various modifications, as aresuited to the particular use contemplated.

What is claimed is:
 1. An article of manufacture, comprising: a silicaencapsulated tin nanoparticle comprising a tin nanoparticle having asilica encapsulating coating disposed thereabout; and a heat transferfluid; wherein the silica encapsulated tin nanoparticle is dispersed inthe heat transfer fluid.
 2. The article of manufacture of claim 1,further comprising a surfactant.
 3. The article of manufacture of claim2 wherein the surfactant is benzalko chloride.
 4. The article ofmanufacture of claim 1, wherein a percentage volume of loading of thesilica encapsulated tin nanoparticle in the heat transfer fluid iswithin the range of 1 vol. % to 5 vol. %.
 5. The article of manufactureof claim 1, wherein the tin nanoparticle has a diameter of between 60 nmand 100nm.
 6. The article of manufacture of claim 1, wherein the silicaencapsulating coating has a thickness of about 5 nm.
 7. The article ofmanufacture of claim 1, further comprising a polyvinylpyrrolidone layerdisposed on the tin nanoparticle.
 8. The article of manufacture of claim1, wherein the tin nanoparticle includes impurities.
 9. A method ofmaking phase change nanoparticles comprising: synthesizing tinnanoparticles by a modified polyole wet-chemical reduction process;encapsulating the tin nanoparticles with a silica encapsulating coating;suspending the encapsulated tin nanoparticles in a heat transfer fluid.10. The method of claim 9, wherein the modified polyole wet-chemicalreduction process comprises: heating a reaction solution containingpolyvinylpyrrolidone (PVP); adding a SnC12 solution to the reactionsolution; adding a NaBH4 solution to the reaction solution, forming thetin nanoparticles; and removing the tin nanoparticles from the reactionsolution.
 11. The method of claim 10, further comprising dissolving thePVP in tetraethylene glycol and heating to about 140 C.
 12. The methodof claim 10, wherein the NaBH₄ solution comprises 15 g:200 ml ratio ofNaBH₄ to tetraethylene glycol.
 13. The method of claim 10, wherein theNaBH4 solution is added drop-wise.
 14. The method of claim 10, whereinthe reaction solution is maintained at about 140 C during addition ofSnC12 and addition of NaBH4 and for 90 minutes thereafter.
 15. Themethod of claim 10, wherein the reaction solution is constantly stirredunder an inert atmosphere.
 16. The method of claim 10, whereinencapsulating the tin nanoparticles comprises base-catalyzed hydrolysisof tetraethyl-ortho-silicate (TEOS).
 17. The method of claim 16, whereinthe base-catalyzed hydrolysis TEOS comprises: suspending the tinnanoparticles in ethanol; sonifying the suspended tin particles;increasing the suspended tin particles to a temperature of 40° C.; andadding the TEOS to the suspended tin particles.
 18. The method of claim17, wherein the TEOS is added dropwise and a resultant mixture isstirred in a darkened environment to prevent photo-polymerization. 19.An article of manufacture, comprising: an encapsulated nanoparticlecomprising a nanoparticle having a ceramic encapsulating coatingdisposed thereabout; and a heat transfer fluid; wherein the ceramicencapsulated nanoparticle is dispersed in the heat transfer fluid. 20.The article of manufacture of claim 19, wherein the ceramic encapsulatednanoparticle comprises a tin nanoparticle.